Acoustic filter



March 15, 1955 w. c. ZAUG 2,704,133

ACOUSTIC FILTER Filed Jan; 26, 1951 FIG. 4.

IN V EN TOR. WILLIAM C. ZAUG TTUEIVEVS BY wwpa Mi M United States PatentACOUSTIC FILTER William C. Zaug, Huntington, N. Y., assignor toTelephonics Corporation, Huntington, N. Y., a corporation of New YorkApplication January 26, 1951, Serial No. 207,932

6 Claims. (Cl. 181-31) This invention relates to acoustic filters, andhas particular reference to damping and protective filters fortelephonic instruments.

The techniques of damping the vibration of a telephonic diaphragm bymeans of a confined air cushion and acoustic shock protection oftelephonic diaphragms by relieving the pressure of a confined aircushion are well known but the duplication of the filter required ineach case has been difiicult and costly on a production basis becausethe precision of manufacture of the filter to make it effective hasrequired much hand work as well as time for testing and adjustment andeven then the percentage of rejected parts due to imperfect filtermanufacture has been disproportionately high.

Inasmuch as both damping and shock protective types of acoustic filtersdepend upon flow of confined air through an orifice, the orifices mustbe the proper size and as the orifices are usually small in area, thenormal manufacturing tolerances afford too wide a variant to produce thecorrect orifice area, and the requisite close tolerances are impracticalfor quantity manufacture. Accordingly, recourse has frequently been hadto the expedient of reducing the effective areas of larger orifices bycovering them with fine screens or plugging them with fibrous filteringmaterial, but both the screen and filtering material absorb or entrainmoisture in use to a degree such that the effective orifice area variesand consequently the acoustic filter characteristics change and areuncertain.

In accordance with the present invention, an acoustic filter and themethod of making the same are provided, wherein the air flow orifice ofthe filter is always of the proper precise area regardless of the numberof telephonic units that are produced embodying the filter, and withoutrequiring the use of uncertain added damping means such as screens,fibrous material, and the like.

In a preferred embodiment of the invention, one wall of the confined aircushion chamber at one or both sides of the diaphragm is provided with alouvre-like slot stamped through the said wall by means of a die whichis precisely made for that purpose. Preferably, the louvre-like slot hasthe same orifice width for the acoustic filters of numerous differenttypes or sizes of telephonic devices requiring acoustic damping oracoustic shock protecting means, and the effective orifice area requiredfor each type or size is simply accommodated by varying the length ofthe slot.

It will be seen that the acoustic filter and the method of making thesame, according to the present invention, is simple but very precise andstill can be reliably reproduced innumerable times without variation,subject only to wear of the die, and is immune to climatic, atmospheric,or other changes in environment, so that its acoustic characteristics donot change.

For a more complete understanding of the invention, reference may be hadto the accompanying drawings, in which:

Figure 1 is the front view of a typical microphone unit embodying theacoustic filter of this invention;

Figure 2 is an axial section therethrough, as seen along the line 2-2 ofFigure 1;

Figure 3 is an enlargement of a portion of the front plate section shownin Figure 2, and illustrates a detail of the louvre-like slot therein;

Figure 4 is an axial section through a typical telephone handsetreceiver unit embodying the acoustic filter of this invention; and

Figure 5 is an enlarged section through the filter as seen along theline 5-5 of Figure 4.

Referring to Figures 1 and 2, and understanding that the microphonedepicted thereby merely illustrates one form of a typical telephonicdevice in which the invention may be utilized to advantage, numeraldesignates the recessed electrode forming with the dome 11 of the thindiaphragm 12, centering washer 13 and filling cap 14, the carbon granulecavity 15. Overlying the diaphragm 12 but spaced therefrom by spacingring 16 is the rigid front plate 17 forming with the diaphragm 12 andspacing ring 16, an air chamber 18. Spacing Washers 19 complete theassembly which is unified by a ring 20 whose crimped or spun over edgesclamp the assembly together.

In order that sound waves may impinge on the dia phragm 12 to enable thediaphragm 12 to respond to speech and accordingly actuate the carbongranules in cavity 15 the front plate 17 is vented in some way, usuallyby perforations of some kind. However, the perforate area frequently isregulated to afford air damping of the vibration of the diaphragm andalso to preclude rupture or other injury to the thin, foil-likediaphragm due to sudden transient pressure shocks such as gun blasts, ofwhich the blast from guns on the order of five inch, and 38 calibre islikely to be the most severe. Accordingly, the diaphragm 12 must beprotected from such transient blasts by an acoustic filter such as anopening in the front plate 17 of an effective area such as to filter outthe sound waves of that magnitude.

The gun blast filter according to this invention is made by stamping aslot 21 in the front plate 17 having the louvre-like shape shown indetail in Figure 3. In the typical case under consideration, the slot 21has an effective width of 0.003 inch and a length of 0.375 inch, givingan effective area of 0.001125 inch.

The dies forming the slot 21 in sheet metal front plate are combinationshearing and forming dies having the contours and the dimensions shownin phantom in Figure 3. 'Thus, assuming that the diaphragm 12 is 0.020brass sheet in the illustrative case, the flat working face of the maledie 22 is provided with a wedge-shaped shearing tongue 23 having alength of 0.376 inch, a height of 0.023 inch, a base width of 0.040inch, and a tip width of 0.010 inch, as shown, with a suitable draftdimension for the shear face to permit separation of the die 22 from theflange or louvre-like stamping 24 without deformation of slot 21 thereofand consequent variation of the effective area of the slot 21. Thefemale die 25 conforms to the other side of the louvre-like stamping 24as shown.

In operation of the filter of Figure 3 as a gun blast filter for amicrophone such as is illustrated in Figures 1 and 2, the slot 21 offersa high impedance to the transient gun blasts which would ordinarilydestroy the instrument by blowing in the diaphragm 12. This highimpedance slot 21 thus limits the air pressure rise in the diaphragmcavity 18 to a point below the rupture pressure of the diaphragm 12 sothat the latter is protected.

As indicated, the slot 21 may perform the function of acousticallydamping the vibration of a diaphragm, such as the diaphragm of atelephone receiver. As an example, Figure 4 shows the louvre-like slotof this invention utilized in a typical telephone handset receivercomprising a cup-like casing containing the permanent magnet 31, voicecoil 32 whose pole shoes are spaced from the imperforate diaphragm 33clamped at its periphery between the rim of the casing 30 and spacingring 34 and overlain by a perforate cap 35 held in place by clampingring 36.

Air-tightly cemented at its periphery to a shoulder 37 in the casing 30and sealed around coil 32 is a baflle plate 38 forming with thediaphragm 33 and the encircling rim of casing 30, the air chamber 39.The bafiie plate may be made of phenolic resin or other relatively rigidnon-magnetic material and is provided with an opening 40 having thecounterbore 41 in which is cemented the acoustic filter disc 42 as isshown in enlarged detail by Figure 5.

Disc 42 may be of 0.005 inch soft brass and is provided with thelouvre-like slot 43 formed therein by a punching die, which maysimultaneously stamp the disc 42 and punch the slot 43 therein. Thepunch forms an 0.0035 inch slit at an angle of 11 under such conditionsthat the material is shaped to leave the desired 0.003 inch slot 43which in this case is 0.250 inch long. Al though the slot 43 has adilferent shape than slot 21 of Figure 3, it performs the same acousticfiltering function and shows that the particular shape of the slot isnot fixed, but may have various shapes, as long as its dimensions arecorrect and are repeated in production without change according to theinvention.

In operation of the acoustic filter of Figure 5 as an acoustic damper inthe handset receiver of Figure 4, for

example, the slot 43 has the effect of regulating the air pressure indiphragm cavity 39 so that the amplitude of the primary resonant peak ofthe diaphragm 33 is reduced and a substantially flat response over thevoice frequency range results. By electrical analogy, the effect issimilar to introducing a resistive component into a resonant circuit,where the resonant element would be analogous to the diaphragm 33 at itsnatural frequency.

Although several preferred embodiments of the invention have beenillustrated and described herein, it is to be understood that theinvention is not limited thereby but is susceptible of changes in formand detail within the scope of the appended claims.

I claim:

1. In an acoustic filter for telephonic instruments having a casingcontaining a diaphragm for receiving or emitting sound waves, thecombination of a wall forming a confined air chamber with saiddiaphragm, and spaced deformations in said wall forming a single slot ofpredetermined dimension for restricting the movement of air in saidconfined chamber, the said spaced deformations being respectivelydisposed along the entire lengths of the longitudinal edges of the slot,with the mutual spacing of the deformations defining the slot area.

2. In an acoustic filter for telephonic instruments having a casingcontaining a diaphragm for receiving or emitting sound waves, thecombination of a wall forming a confined air chamber with saiddiaphragm, and a louvre-shaped deformation in said wall forming a slotof predetermined dimension for restricting the movement of air in saidconfined chamber, said deformation in the wall being integral with thewall along the entire length of the slot, thereby to define a singleslot.

3. In an acoustic filter for telephonic instruments having a casingcontaining a diaphragm for receiving or emitting sound waves, thecombination of a wall forming a confined air chamber with saiddiaphragm, and a deformed portion on said wall forming a slot ofpredetermined dimension for restricting the movement of air in saidconfined chamber, said slot being longer than it is wide and saiddeformed portion being substantially rectangular and formed integrallywith the wall along the entire length of only one'side of the slot, thenarrow dimension of said deformed portion forming an angle with theplane of the wall.

4. In an acoustic filter for telephonic instruments having a casingcontaining a diaphragm for receiving or emitting sound waves, thecombination of a wall forming a confined air chamber with saiddiaphragm, and a single deformed slit in said wall forming a single slotof predetermined dimension for restricting the movement of air in saidconfined chamber.

5. In an acoustic filter for telephonic instruments having a casingcontaining a diaphragm for receiving or emitting sound waves, thecombination of a wall forming a confined air chamber with saiddiaphragm, said wall having a single slit, and a deformed longitudinaledge of said slit forming with the opposite edge of said slit a narrowslot of predetermined dimension for restricting the movement of air insaid confined chamber.

6. In an acoustic filter for telephonic instruments having a casingcontaining a diaphragm for receiving or emitting sound waves, thecombination of a wall forming a confined air chamber with saiddiaphragm, a deformable portion in said wall, and a single deformed slitthrough said portion forming a single slot of predetermined dimensionfor restricting the movement of air in said confined chamber.

References Cited in the file of this patent UNITED STATES PATENTS1,438,968 Sampson Dec. 19, 1922 1,637,322 Weil July 26, 1927 1,709,276Mourar Apr. 16, 1929 1,715,831 Hahnemann June 4, 1929 1,737,346 WilckensNov. 26, 1929 1,951,874 Kellar Mar. 20, 1934 1,991,748 Johnson Feb. 19,1935 2,529,467 Wiggins Nov. 7, 1950 2,537,253 Anderson Jan. 9, 19512,557,687 Rainville June 19, 1951

